An instrument, used with a telescope or microscope, for measuring minute distances, or the apparent diameters of objects which subtend minute angles. The measurement given directly is that of the image of the object formed at the focus of the object glass.

Circular, ∨ Ring, micrometer, a metallic ring fixed in the focus of the object glass of a telescope, and used to determine differences of right ascension and declination between stars by observations of the times at which the stars cross the inner or outer periphery of the ring. -- Double image micrometer, a micrometer in which two images of an object are formed in the field, usually by the two halves of a bisected lens which are movable along their line of section by a screw, and distances are determined by the number of screw revolutions necessary to bring the points to be measured into optical coincidence. When the two images are formed by a bisected objects glass, it is called a divided-object-glass micrometer, and when the instrument is large and equatorially mounted, it is known as a heliometer. -- Double refraction micrometer, a species of double image micrometer, in which the two images are formed by the double refraction of rock crystal. -- Filar, ∨ Bifilar, micrometer. See under Bifilar. -- Micrometer caliper ∨ gauge] Mech., a caliper or gauge with a micrometer screw, for measuring dimensions with great accuracy. -- Micrometer head, the head of a micrometer screw. -- Micrometer microscope, a compound microscope combined with a filar micrometer, used chiefly for reading and subdividing the divisions of large astronomical and geodetical instruments. -- Micrometer screw, a screw with a graduated head used in some forms of micrometers. -- Position micrometer. See under Position. -- Scale, ∨ Linear, micrometer, a minute and very delicately graduated scale of equal parts used in the field of a telescope or microscope, for measuring distances by direct comparison.